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Water Sources and Water Treatment Drinking water should be essentially free of disease-causing microbes, but often this is not the case. –A large proportion of the world’s population drinks microbially contaminated water, especially in developing countries Using the best possible source of water for potable water supply and protecting it from microbial and chemical contamination is the goal –In many places an adequate supply of pristine water or water that can be protected from contamination is not available The burden of providing microbially safe drinking water supplies from contaminated natural waters rests upon water treatment processes –The efficiency of removal or inactivation of enteric microbes and other pathogenic microbes in specific water treatment processes has been determined for some microbes but not others. – The ability of water treatment processes and systems to reduce waterborne disease has been determined in epidemiological studies

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Coagulation-Flocculation, Continued Flocculation: Slow mixing (flocculation) that provides for for a period of time to promote the aggregation and growth of the insoluble particles (flocs). The particles collide, stick together abd grow larger The resulting large floc particles are subsequently removed by gravity sedimentation (or direct filtration) Smaller floc particles are too small to settle and are removed by filtration

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Microbe Reductions by Chemical Coagulation- Flocculation Considerable reductions of enteric microbe concentrations. Reductions In laboratory and pilot scale field studies: – >99 percent using alum or ferric salts as coagulants – Some studies report much lower removal efficiencies (<90%) – Conflicting information may be related to process control coagulant concentration, pH and mixing speed during flocculation. Expected microbe reductions bof 90-99%, if critical process variables are adequately controlled No microbe inactivation by alum or iron coagulation – Infectious microbes remain in the chemical floc – The floc removed by settling and/or filtration must be properly managed to prevent pathogen exposure. Recycling back through the plant is undesirable Filter backwash must be disinfected/disposed of properly.

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Slow Sand Filters Less widely used for large US municipal water supplies Effective; widely used in Europe; small water supplies; developing countries Filter through a 3 ‑ to 5 ‑ foot deep bed of unstratified sand flow rate ~0.05 gallons per minute per square foot. Biological growth develops in the upper surface of the sand is primarily responsible for particle and microbe removal. Effective without pretreatment of the water by coagulation ‑ flocculation Periodically clean by removing, cleaning and replacing the upper few inches of biologically active sand

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Properties of an Ideal Disinfectant Broad spectrum: active against all microbes Fast acting: produces rapid inactivation Effective in the presence of organic matter, suspended solids and other matrix or sample constituents Nontoxic; soluble; non-flammable; non-explosive Compatible with various materials/surfaces Stable or persistent for the intended exposure period Provides a residual (sometimes this is undesirable) Easy to generate and apply Economical

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Disinfection Kinetics Disinfection is a kinetic process Increased inactivation with increased exposure or contact time. –Chick's Law: disinfection is a first ‑ order reaction. (NOT!) –Multihit-hit or concave up kinetics: initial slow rate; multiple targets to be “hit” –Concave down or retardant kinetics: initial fast rate; decreases over time Different susceptibilities of microbes to inactivation; heterogeneous population Decline of of disinfectant concentration over time CT Concept: Disinfection can be expressed at the product of disinfectant concentration X contact time –Applies best when disinfection kinetics are first order Disinfectant concentration and contact time have an equal effect on CT products Applies less well when either time ofrconcentration is more important.

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Types of Disinfection Kinetics Disinfection is a kinetic process Increased inactivation with increased exposure or contact time. – Chick's Law: disinfection is a first ‑ order reaction. (NOT!) – Multihit-hit or concave up kinetics: initial slow rate; multiple targets to be “hit”; diffusion-limitions in reaching “targets” – Concave down or retardant kinetics: initial fast rate that decreases over time Different susceptibilities of microbes to inactivation; heterogeneous population Decline of of disinfectant concentration over time

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Disinfection Activity and the CT Concept Disinfection activity can be expressed as the product of disinfection concentration (C) and contact time (T) Assumes first order kinetics (Chick’s Law) such that disinfectant concentration and contact time have the same “weight” or contribution in disinfection activity and in contributiong to CT Example: If CT = 100 mg/l-minutes, then –If C = 10 mg/l, T must = 10 min. in order to get CT = 100 mg/l-min. –If C = 1 mg/l, then T must = 100 min. to get CT = 100 mg/l-min. –If C = 50 mg/l, then T must = 2 min. to get CT = 100 mg/l-min. –So, any combinationof C and T giving a product of 100 is acceptable because C and T are interchangable The CT concept fails if disinfection kinetics do not follow Chick’s Law (are not first-order or exponential)

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Free Chlorine - Background and History Considered to be first used in 1905 in London – But, electrochemically generated chlorine from brine (NaCl) was first used in water treatment the late 1800s Reactions for free chlorine formation: Cl 2 (g) + H2O HOCl + H+ + Cl- HOCl H + + OCl - Chemical forms of free chlorine: Cl 2 (gas), NaOCl (liquid), or Ca(OCl) 2 (solid) Has been the “disinfectant of choice” in US until recently. recommended maximum residual concentration of free chlorine < 5 mg/L (by US EPA) Concerns about the toxicity of free chlorine disinfection by- products (trihalomethanes and other chlorinated organics)

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Free Chlorine and Microbial Inactivation Greater microbial inactivation at lower pH (HOCl) than at high pH (OCl - ) – Probably due to greater reactivity of the neutral chemical species with the microbes and its constituents Main functional targets of inactivation: – Bacteria: respiratory activities, transport activities, nucleic acid synthesis. – Viruses: reaction with both protein coat (capsid) and nucleic acid genome – Parasites: mode of action is uncertain Resistance of Cryptosporidium to free chlorine (and monochloramine) has been a problem in drinking water supplies – Free chlorine (bleach) is actually used to excyst C. parvum oocysts!

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Monochloramine - History and Background First used in Ottawa, Canada and Denver, Co. (1917) Became popular to maintain a more stable chlorine residual and to control taste and odor problems and bacterial re-growth in distribution system in 1930’s Decreased usage due to ammonia shortage during World War II Increased interest in monochloramine: – alternative disinfectant to free chlorine due to low THM potentials – more stable disinfectant residual; persists in distribution system – secondary disinfectant to ozone and chlorine dioxide disinfection to provide long-lasting residuals

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Reaction of Ammonia with Chlorine: Breakpoint Chlorination Presence of ammonia in water or wastewater and the addition of free chlorine results in an available chlorine curve with a “hump” At chlorine doses between the hump and the dip, chloramines are being oxidatively destroyed and nitrogen is lost (between pH 6.5-8.5). Chlorine added, mg/L Cl 2 avail. @ 30 min., mg/L Combined Cl 2 present Free chlorine present

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Ozone First used in 1893 at Oudshoon Used in 40 WTPs in US in 1990 (growing use since then), but more than 1000WTPs in European countries Increased interest as an alternative to free chlorine (strong oxidant; strong microbiocidal activity; perhaps less toxic DBPs) – A secondary disinfectant giving a stable residual may be needed to protect water after ozonation, due to short-lasting ozone residual. Colorless gas; relatively unstable; reacts with itself and with OH - in water; less stable at higher pH Formed by passing dry air (or oxygen) through high voltage electrodes to produce gaseous ozone that is bubbled into the water to be treated.

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Chlorine Dioxide First used in Niagara Fall, NY in 1944 to control phenolic tastes and algae problems Used in 600 WTP (84 in the US) in 1970’s as primary disinfectant and for taste and odor control Very soluble in water; generated as a gas or a liquid on-site, usually by reaction of Cl 2 gas with NaClO 2 : – 2 NaClO 2 + Cl 2  2 ClO 2 + 2 NaCl Usage became limited after discovery of it’s toxicity in 1970’s & 1980’s – thyroid, neurological disorders and anemia in experimental animals by chlorate Recommended maximum combined concentration of chlorine dioxide and it’s by-products < 0.5 mg/L (by US EPA in 1990’s)

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Chlorine Dioxide High solubility in water – 5 times greater than free chlorine Strong Oxidant; high oxidative potentials; – 2.63 times greater than free chlorine, but only 20 % available at neutral pH Neutral compound of chlorine in the +IV oxidation state; stable free radical – Degrades in alkaline water by disproportionating to chlorate and chlorite. Generation: On-site by acid activation of chlorite or reaction of chlorine gas with chlorite About 0.5 mg/L doses in drinking water – toxicity of its by-products discourages higher doses